Add documentation for passphrase option.

The wifi-iface actually supports an undocumented option to choose
whether to treat a passphrase as a text passphrase or an encoded
passphrase (like encoded by the wpa_passphrase utility). This patch
documents that functionality.

Signed-off-by: Warren Turkal <wt@penguintechs.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@17712 3c298f89-4303-0410-b956-a3cf2f4a3e73